l antecedent and its consequent seem
_conceivable_, has perpetually varied, since it depends on a person's
special habits of thought. Thus, the Greeks, Thales, Anaximenes, and
Pythagoras, thought respectively that water, air, or number is such an
agency explaining the production of physical effects. Many moderns,
again, have been unable to _conceive_ the production of effects by
volition itself, without some intervening agency to connect it with
them. This medium, Leibnitz thought, was some _per se_ efficient
physical antecedent; while the Cartesians imagined for the purpose the
theory of Occasional Causes, that is, supposed that God, not _qua_ mind,
or _qua_ volition, but _qua_ omnipotent, intervenes to connect the
volition and the motion: so far is the mind from being forced to think
the action of mind on matter more _natural_ than that of matter on
matter. Those who believe volition to be an efficient cause are guilty
of exactly the same error as the Greeks, or Leibnitz or Descartes; that
is, of requiring an _explanation_ of physical sequences by something
[Greek: aneu hou to aition ouk an pot' eie aition]. But they are guilty
of another error also, in inferring that volition, even if it is an
_efficient_ cause of so peculiar a phenomenon as nervous action, must
therefore be the efficient cause of all other phenomena, though having
scarcely a single circumstance in common with them.

CHAPTER VI.

THE COMPOSITION OF CAUSES.

An effect is almost always the result of the concurrence of several
causes. When all have their full effect, precisely as if they had
operated _successively_, the joint effect (and it is not inconsistent to
give the name of _joint effect_ even to the mutual obliteration of the
separate ones) may be _deduced_ from the laws which govern the causes
when acting separately. Sciences in which, as in mechanics, this
principle, viz. the _composition of causes_, prevails, are deductive and
demonstrative. Phenomena, in effect, do generally follow this principle.
But in some classes, e.g. chemical, vital, and mental phenomena, the
laws of the elements when called on to work together, cease and give
place to others, so that the joint effect is not the sum of the separate
effects. Yet even here the more general principle is exemplified. For
the new _heteropathic_ laws, besides that they never supersede _all_ the
old laws (thus, The weight of a chemical compound is equal to the sum of
the weight of t